Coaxial cable is in widespread use for transmitting signals, particularly over cable television lines. A coaxial cable has a center conductor that is insulated. An outer conductor, which may be foil, woven, or multiple layers of both, surrounds the inner conductor insulation. Although is has been a longstanding practice to provide a protective device at the junction box between the telephone company lines, which are twisted-pairs, and equipment in a home or business, this has not been widely used with coaxial distribution lines. For telephone cable lines, the protection device includeds an excess voltage protector that conducts to a ground when encountering excessive voltage. The excess voltage protector may be of a gas tube type or solid state. For excess current protection, a fuse will be provided.
Overcurrent protection devices have been used to some extent for coaxial cables. One prior practice has been to connect into the line a relatively long length, approximately 20 inches, of coaxial cable with a center conductor wire that has a gage two or three sizes smaller than the gage of the network center conductor wire. This technique is not completely reliable as the coaxial cable intended to be a fuse link does not always open in a predictable location. For example, the coaxial connector may inadvertently act as the fusible element, which is unsatisfactory. Another technique is to use a medium length coaxial cable, less than three inches, which has been designed with an extremely small gage center wire. This particular type is difficult to manufacture. Available overcurrent protection devices are usually contained in a physically separate package from overvoltage protection devices.
Another problem dealing with protection devices involves characteristic impedance mismatch. It is important to match the characteristic impedance of the protection device to the characteristic impedance of the transmission line, which in the case of coaxial cable for cable television applications is typically 75 ohms. Impedance mismatch may result in unacceptable insertion loss and return loss characteristics, which results in data loss. Overvoltage protection elements, such as air gaps, gas tubes, or solid state devices such as thyristors, have a capacitance that is often many magnitudes larger than the inherent capacitance of the transmission line in the network they are designed to protect. When these devices are inserted into the transmission line, the characteristic impedance of the network becomes mismatched in the area of the protector and signal losses occur.